Process for separating arsenic acid from an aqueous mixture comprising sulfuric and arsenic acids

Abstract: The present invention provides a process for recovering arsenic acid from a starting mixture comprising sulfuric and arsenic acids and water. In step (a), the starting mixture is treated with a sulfur (IV) compound which will reduce the arsenic acid to arsenic (III) compound under conditions sufficient to substantially convert the arsenic acid to arsenic (III) wherein the resulting mixture comprises arsenic (III) compound, the sulfur (IV) compound, sulfuric acid, and water. In step (b), the resulting mixture is purged with inert gas to substantially remove the sulfur (IV) compound from the mixture wherein the purged mixture comprises the arsenic (III) compounds, sulfuric acid, and water. In step (c), the purged mixture is treated under conditions sufficient to substantially separate the arsenic (III) compounds from the purged mixture. In step (d), the separated arsenic (III) compound is reacted with an oxidizing agent to substantially convert arsenic (III) compound to arsenic acid wherein the final mixture comprises arsenic acid, unreacted oxidizing agent, and water. In step (e), impurities are removed from the final mixture to provide substantially pure aqueous arsenic acid. The recovered arsenic acid may be used for example as a raw material for the production of CCA wood preservative. The purged mixture which is sulfuric acid may be reused for example in the process of commonly assigned U.S. Patent 5,089,241.

Description: BACKGROUND OF THE INVENTION The present invention relates to a process for the separation of aqueous mixtures of arsenic acid or salts thereof from a waste mixture comprising sulfuric and arsenic acids and producing a high purity arsenic acid product. Commonly assigned U.S. Patent 5,089,241 describes a process for the conversion of hazardous hexafluoroarsenic acid or any salt thereof to arsenic acid. The resulting hazardous mixture comprises about 45 to about 85 weight percent sulfuric acid, about 5 to about 25 weight percent arsenic acid, and about 15 to about 40 weight percent water. The patent teaches that this hazardous waste may be rendered non-hazardous by the use of known methods. Rather than generate waste for storage in a landfill, a process is needed which separates the arsenic acid from the waste mixture such that the separated arsenic acid may be reused. A process which also separates the sulfuric acid such that the separated sulfuric acid also could be reused would be extremely advantageous. Attempts have been made to solve this problem of separating the arsenic acid or salts thereof from sulfuric acid in various media but the following problems have been encountered. L. Nan et al., "Kinetics of Reduction of Arsenic (V) to Arsenic (III) with Sulfur Dioxide in Aqueous Solution", Australas. Inst. Min. Metall. Publ. Ser. 1989, 6/89 (Non-ferrous Smelting Symp., 1989, 145-52) report that the reduction of arsenic acid to arsenious acid with sulfur dioxide in aqueous solution proceeds in two steps: the absorption of the sulfur dioxide from the gas phase into the aqueous phase and the reduction of the arsenic acid. Sulfuric acid was initially added to the arsenic acid to adjust pH. The weight percent of arsenic acid used was 1 to 14 while the weight percent of sulfuric acid used was 0.5 to 30. Most of the experiments were conducted with 0.01 M or 0.1 M initial arsenic acid and a small amount of sulfuric acid added to adjust the pH to about 4. The reference reports that when the initial arsenic acid was then increased to 1 M, the arsenic reduction rate decreased by about 30 percent. The reference teaches that the reaction rate was considerably reduced in concentrated sulfuric acid solutions. Thus, the reference discourages the use of a starting material having higher concentrations of sulfuric and arsenic acids therein. Further, the reference teaches that the arsenious acid is the final product and not further reacted. See also K. Gritton et al., "Metal Recovery from Metallurgical Wastes", Society for Mining, Metallurgy and Exploration, Inc , February 26-March 1, 1990. East German Patent 248,249-A3 dated August 5, 1987 teaches a process for separating arsenic trifluoride from a product mixture containing hydrogen fluoride. The process involves reacting the mixture of arsenic trifluoride and hydrogen fluoride with arsenic trioxide and sulfuric acid wherein the hydrogen fluoride reacts with the arsenic trioxide to form arsenic trifluoride. The reference teaches that the resulting arsenic trifluoride may be used as an implantation gas in the microelectronics industry and does not teach that the arsenic trifluoride is further reacted. GB-A-1444058 relates to a process for the recovery of trivalent arsenic (as As 2 O 3 ) from sulfuric acid solution containing other dissolved metals in addition to arsenic, wherein the arsenic is reduced from As V to As III by means of sulfur dioxide, under super-atmospheric pressure at a temperature of 100° to 150°C, the molar ratio of sulfur dioxide to pentavalent arsenic present being between 1.05 and 1.30. Any other dissolved metals are removed by precipitation as sulfates before or after the reduction of the arsenic in solution. Preferably the arsenic trioxide is precipitated and recovered by evaporating the solution to a sulfuric acid concentration of 50% or above. US-A-4220627 relates to a process for the treatment of raw materials which contain arsenic and metal to produce a metal-free arsenic product and an arsenic-free metal sulfate, by leaching the raw material with an aqueous solution of sulfuric acid at elevated temperature under oxidizing conditions; selectively separating an arsenate-bearing aqueous solution; cooling the solution to crystallize the metals as sulfates from the separated aqueous solution; separating the aqueous solution from the metal sulfate crystals; removing arsenic from at least part of the aqueous solution, and finally recycling the solution to the leach. The arsenic can be removed by solvent extraction or by reduction of the arsenate to arsenite by means of sulfur trioxide. US-A- 4,891,207 teaches the oxidation of arsenic trioxide with hydrogen peroxide to form arsenic acid. The reference does not teach how to separate arsenic acid from a mixture containing sulfuric acid. As such, the need exists in the art for a process for separating arsenic acid from an aqueous waste mixture comprising sulfuric and arsenic acids. We have solved this problem in the art by developing a process for separating the aqueous arsenic acid from a waste mixture comprising sulfuric and arsenic acids and producing a pure arsenic acid product. One embodiment of the present invention provides a process for recovering arsenic acid from a starting mixture comprising sulfuric and arsenic acids and water comprising the steps of: (a) treating said starting mixture comprising at least 1 weight percent of arsenic acid, at least 1 weight percent of sulfuric acid and up to 98 weight percent of water, based on the total amount of said starting mixture, with a sulfur (IV) compound which will reduce the arsenic acid to arsenic (III) compound, under conditions sufficient to substantially convert said arsenic acid into arsenic (III) compounds, sufficient hydrogen fluoride being present in said starting mixture to substantially convert the arsenic (III) compound into arsenic trifluoride, and to provide a resulting mixture comprising said arsenic trifluoride, said sulfur (IV) compound, said sulfuric acid, and said water; (b) purging said resulting mixture with inert gas to substantially remove said sulfur (IV) compound from said mixture to provide a purged mixture comprising said arsenic trifluoride, said sulfuric acid, and said water; (c) adding sulfuric acid to said purged mixture, in an amount sufficient and at a temperature sufficient to substantially volatilize said arsenic trifluoride, wherein said volatilization substantially separates said arsenic trifluoride from said purged mixture; (d) reacting said separated arsenic trifluoride with oxidizing agent under conditions sufficient to substantially convert said arsenic trifluoride into arsenic acid to produce a final mixture comprising said arsenic acid, unreacted oxidizing agent, and said water; and (e) removing impurities from said final mixture, wherein said impurities comprise said unreacted oxidizing agent, to provide substantially pure aqueous arsenic acid. An alternative embodiment of the present invention provides a process for recovering arsenic acid from a starting mixture comprising sulfuric and arsenic acids and water comprising the steps of: (a) treating said starting mixture comprising at least 1 weight percent of arsenic acid, at least 1 weight percent of sulfuric acid and up to 98 weight percent of water, based on the total amount of said starting mixture, with a sulfur (IV) compound which will reduce the arsenic acid to arsenic (III) compound, under conditions sufficient to substantially convert said arsenic acid into arsenic (III) compounds and to provide a resulting mixture comprising said arsenic (III) compound, said sulfur (IV) compound, said sulfuric acid, and said water; (b) purging said resulting mixture with inert gas to substantially remove said sulfur (IV) compound from said mixture to provide a purged mixture comprising said arsenic (III) compound, said sulfuric acid, and said water; (c) reacting said purged mixture with hydrogen fluoride, in an amount sufficient to substantially convert the arsenic (III) compound into arsenic trifluoride, and adding sulfuric acid to said purged mixture in an amount sufficient to substantially volatilize said arsenic trifluoride thereby substantially separating said arsenic trifluoride from said purged mixture. (d) reacting said separated arsenic trifluoride with oxidizing agent under conditions sufficient to substantially convert said arsenic trifluoride into arsenic acid to produce a final mixture comprising said arsenic acid, unreacted oxidizing agent, and said water; and (e) removing impurities from said final mixture, wherein said impurities comprise said unreacted oxidizing agent, to provide substantially pure aqueous arsenic acid. (The term "arsenic (III) compound" as used herein means at least one of the following trivalent arsenic forms: H 3 AsO 3 , HAsO 2 , As 2 O 3 , and AsF 3 ). The present process is advantageous because it recovers arsenic acid and thus, eliminates the current need for converting the hazardous arsenic waste to non-hazardous waste, stabilizing the waste, and landfilling it. The present process also separates sulfuric acid. The separated arsenic and sulfuric acids may then be reused. Other advantages of the present invention will be apparent from the following description and attached claims. The Figure is a schematic of the present invention. The present invention provides a process for separating arsenic acid from a waste mixture comprising sulfuric and arsenic acids and water. Such a waste mixture may result from a process such as that disclosed by US-A-5,089,241. Such waste mixtures may also contain acids such as hydrofluoric, phosphoric, fluosilicic, and fluosulfonic. Generally speaking, this mixture is considered poisonous and carcinogenic because the waste contains arsenic compounds and is corrosive because the pH is less than 2. In the United States for disposal of this waste, the waste must be treated such that the stabilized waste passes the Toxic Characteristics Leach Procedure Test (TCLP) established by the Environmental Protection Agency in order to be considered nonhazardous. Other countries have comparable regulations also. In step (a), the starting mixture is treated with a sulfur (IV) compound under conditions sufficient to substantially convert the arsenic acid to arsenic (III) compound wherein the resulting mixture comprises arsenic (III) compound, sulfur (IV) compound, sulfuric acid, a small amount of unconverted arsenic acid, and water. We have discovered that in step (a),the conversion of arsenic (V) to arsenic (III) relates to the following conditions: (1) initial arsenic acid and sulfuric acid concentration; (2) water concentration in the mixture to be reacted; (3) reactor pressure; (4) degree of agitation and therefore the gas/liquid contact surface area; (5) the reaction time; and (6) the reaction temperature. The starting mixture comprises at least 1 weight percent arsenic acid, at least 1 weight percent sulfuric acid, and up to 98 weight percent water based on the total amount of the starting mixture. Preferably, the starting mixture comprises at least 9 weight percent arsenic acid and at least 60 weight percent sulfuric acid. A water content of at least 30 weight percent is preferred because the conversion of the pentavalent arsenic to trivalent arsenic increases and thus reaction time decreases. We have found that water content greater than 30 weight percent does not significantly increase the conversion of arsenic acid to arsenic (III) compound in step (a). In a hydrogen fluoride manufacturing facility, a higher water content may be detrimental because the excess water must be reacted with oleum to enable recycling of the arsenic depleted sulfuric acid. As indicated above, significantly lower water content decreases the conversion and is neither economical nor advantageous because more unreacted arsenic acid has to be recycled. We have also found that as the pressure increases, the solubility of the sulfur (IV) compound in the starting mixture increases. As the concentration of sulfur (IV) compound in the starting mixture increases, the conversion of arsenic acid to arsenic (III) compound increases and the reaction time decreases. We have found that pressures less than 237.9 kPa (20 psig) do not have a significant effect on the conversion or reaction time. Preferably, the sulfur (IV) compound is introduced under a pressure greater than 237.9 kPa (20 pounds per square inch) into the starting mixture in step (a). The more preferred pressure range is 237.9 to 341.3 kPa (20 to 35 psig). We have also found that as the degree of contact between the liquid and gas phases in the reactor increases, the solubility of the sulfur (IV) compound in the starting mixtures increases. Agitation is one means of increasing the effective surface area between the reactants. We have found that agitation increases the surface contact of the gas and liquid phases and therefore, the conversion increases. The gas and liquid phases surface contact may be accomplished by employing any known agitation means. Thus preferably, the mixture is agitated in an amount sufficient to produce intimate contact between the starting mixture and the sulfur (IV) compound in step (a). A packed recirculating column may be useful for this purpose. We have also found that as the reaction time increases, the conversion increases. The preferred reaction time for step (a) is at least about two hours. Reaction times greater than about four hours did not significantly increase the conversion. The more preferred reaction time for step (a) is about two to about four hours. We have also found that as the reaction temperature increases, the solubility of the sulfur (IV) compound decreases and as the temperature decreases significantly, the reaction kinetics limit the reaction rate. Preferably, the reaction temperature for step (a) is about 20°C to about 30°C. Preferably, the sulfur (IV) compound is selected from the group consisting of sulfur dioxide gas or liquid, aqueous sulfur dioxide (sulfurous acid), and bisulfite and sulfite salts such as sodium, potassium, or ammonium. The more preferred sulfur (IV) compound is sulfur dioxide. If sulfur dioxide is used in step (a), the following reaction occurs: H 3 AsO 4 + SO 2 → HAsO 2 + H 2 SO 4 As the foregoing reduction proceeds, the HAsO 2 arsenious acid, becomes insoluble in the reaction mixture and forms arsenic trioxide solids according to the following reaction: 2HAsO 2 → As 2 O 3 + H 2 O Referring to the Figure which is meant to be illustrative and non-limiting, the waste mixture as indicated by arrow 12 and sulfur (IV) compound as indicated by arrow 14 may be fed into reactor 16. Preferably, the waste mixture is agitated in order to increase the contact between the sulfur (IV) compound and the waste mixture so as to substantially convert the arsenic acid to arsenic (III) compound and to produce a mixture comprising arsenic (III) compound, the sulfur (IV) compound, sulfuric acid, unconverted arsenic acid, and water. The term "arsenic III compound" as used herein means one or more of the following trivalent arsenic forms: H 3 AsO 3 , HAsO 2 , As 2 O 3 , and AsF 3 . Reactor 16 should be constructed of a material which is not attacked by the corrosive waste mixture so as to preclude contamination and ensure equipment longevity. Accordingly, all surfaces of reactor 16 which come into contact with the corrosive waste mixture should be inert to the corrosive waste mixture. The excess sulfur (IV) compound in step (a) may be vented through arrow 18 to scrubber 20 and the purged gas is vented through arrow 22. The excess sulfur (IV) compound may also be recovered instead of scrubbed and reused in the present process. In order to ultimately generate arsenic and sulfuric acids which are free of sulfur (IV) compound, the mixture resulting from step (a) is purged with inert gas in step (b) so as to substantially remove the sulfur (IV) compound from the mixture wherein the purged mixture comprises the arsenic (III) compounds, sulfuric acid, unconverted arsenic acid, and water. Any inert gas which is capable of substantially removing all of the sulfur (IV) compound and does not oxidize the arsenic (III) compound back to arsenic acid may be used for step (b). Examples of useful inert gases include air, nitrogen, steam, carbon dioxide, and methane. The preferred inert gas is air. Mixtures of inert gases may also be used. Preferably, the sulfur (IV) compound is substantially removed as sulfur dioxide gas. Referring to the Figure, inert gas may be introduced through arrow 24 into the reaction mixture in reactor 16. The excess inert gas may also be vented through arrow 18 to scrubber 20 and vented through arrow 22. Step (c) comprises treating the purged mixture under conditions sufficient to substantially separate the arsenic (III) from the purged mixture in the form of arsenic trifluoride. Hydrogen fluoride is added to the starting mixture in step (a) or to the purged mixture of step (c) in an amount sufficient to substantially convert the arsenic (III) compound into arsenic trifluoride. Preferably, the amount of hydrogen fluoride added is at least twice the weight percent of the arsenic (III) compound. This process is advantageous because it involves a liquid stream which avoids the hygienic and environmental problems associated with separating a solid phase. Thus, in step (c), the following reactions occur in the preferred embodiment: HAsO 2 + 3HF → AsF 3 ↑ + 2H 2 O As 2 O 3 + 6HF → 2AsF 3 ↑ + 3H 2 O Sulfuric acid is added in step (c) in an amount sufficient and at a temperature sufficient to substantially volatilize the arsenic trifluoride wherein the volatilization substantially separates the arsenic trifluoride from the purged mixture. Sulfuric acid is added as necessary to the mixture resulting from step (b) in order to decrease the solubility of the formed arsenic trifluoride in the resulting mixture. The sulfuric acid may be added to the mixture resulting from step (b) before or after the hydrogen fluoride is added or simultaneously with the hydrogen fluoride addition. The heat of dilution of sulfuric acid raises the mixture temperature. Any commercially available heating means may also be used to raise the mixture temperature so as to substantially volatilize the formed arsenic trifluoride. Preferably, the amount of sulfuric acid added is at least about 50 percent by weight based on the total amount of the purged mixture and the temperature is at least about 120°C. More preferably, the temperature is about 120°C to about 130°C. Referring to the Figure, the hydrogen fluoride may be fed as indicated by arrow 26 into the reactor 16. The volatilized arsenic trifluoride may then be fed from reactor 16 as indicated by arrow 28 into vessel 30. The volatilization of the arsenic trifluoride separates the arsenic trifluoride from the mixture. The residual mixture in reactor 16 comprises sulfuric acid and a small portion of the starting arsenic acid and water. This sulfuric acid mixture may then be reused. As an example, the sulfuric acid may be reused in the process of US-A-5,089,241. Thus, the present process is advantageous in separating arsenic acid from sulfuric acid such that the sulfuric acid may be reused. In step (d), the separated arsenic trifluoride is reacted with an oxidizing agent under conditions sufficient to substantially convert the arsenic trifluoride to arsenic acid wherein the final mixture comprises arsenic acid, unreacted oxidizing agent, and water. Step (d), comprises the formation of the arsenic acid product from the arsenic trifluoride separated by volatilization in step (c) with a suitable oxidizing agent such as hydrogen peroxide. The reaction of the arsenic trifluoride occurs rapidly at room temperature with stoichiometric quantities of reactants. To ensure complete conversion to arsenic acid occurs, a 10% excess of oxidizing agent may be added. Commercially available hydrogen peroxide may be used for this reaction. The resulting solution from step (d) contains arsenic acid, water, hydrogen fluoride, and undecomposed oxidizing agent. Any oxidizing agent which absorbs the arsenic trifluoride oxidises it to arsenic acid, and does not contaminate the arsenic acid may be used. Examples of useful oxidizing agents include commercially available hydrogen peroxide, or nitric acid. Nitric acid requires the use of a catalyst such as potassium iodide. The preferred oxidizing agent is hydrogen peroxide because hydrogen peroxide reacts rapidly, decomposes readily, does not require a catalyst, and does not impart any impurity into the arsenic acid product. The reaction between arsenic trifluoride and hydrogen peroxide occurs instantaneously at room temperature. If hydrogen peroxide is used in step (d) as the oxidizing agent, the following reactions occur: AsF 3 + 3H 2 O 2 → H 3 AsO 4 + 3HF + O 2 ↑ 2H 2 O 2 → 2H 2 O + O 2 Referring to the Figure, the volatilized arsenic trifluoride may be fed into vessel 30 which contains the oxidizing agent. Vessel 30 should be constructed of a material which is not attacked by the oxidizing agent or the acid solution so as to preclude contamination and ensure equipment longevity. Accordingly, all surfaces of the vessel 30 which come into contact with the oxidizing agent must be inert to the oxidizing agent and the acid solution so as not to contaminate the final arsenic acid product. The solution resulting from step (d) comprises arsenic acid, water, hydrogen fluoride, and unreacted oxidizing agent. Step (e) comprises removing impurities from the final mixture, wherein the impurities comprise unreacted oxidizing agent, to provide substantially pure aqueous arsenic acid. If the final mixture further comprises hydrogen fluoride, the impurities further comprise hydrogen fluoride. Preferably, step (e) comprises heating the final mixture at a temperature sufficient to substantially decompose unreacted oxidizing agent. Preferably, the final mixture is heated at a temperature sufficient to substantially decompose the unreacted oxidizing agent. More preferably, the final mixture is heated at 120°C to 130°C for 15 to 30 minutes. The hydrogen fluoride is removed by breaking the hydrogen fluoride - water azeotrope. Preferably in step (e), the final mixture is heated at a temperature sufficient to substantially remove the hydrogen fluoride and water and an inert gas is passed through the heated mixture in a quantity sufficient to substantially remove the hydrogen fluoride in the heated mixture from the heated mixture. This may be accomplished by concentrating the mixture to give an arsenic acid concentration greater than about 50 percent by weight and stripping with an inert gas such as air or steam. A portion of the purified arsenic acid may be recycled back to this step as required to break the hydrogen fluoride - water azeotrope. As taught by commonly assigned U.S.Patent 5,089,241, hexafluoroarsenic acid may be formed by the following reaction: H 3 AsO 4 + 6HF → HAsF 6 + 4H 2 O and thus, be present in the mixture. In order to hydrolyze the hexafluoroarsenic acid to arsenic acid, substantially all of the hydrogen fluoride must be removed. In order to substantially hydrolyze the hexafluoroarsenic acid to arsenic acid, the amount of water present has to be sufficient. Additionally, the water prevents the arsenic acid from solidifying. Otherwise, arsenic acid which is concentrated at greater than 80% solidifies easily to form solid hydrates of As 2 O 5 . Although hexafluoroarsenic acid will be partially converted to arsenic acid in the presence of hydrogen fluoride, substantially all of the hydrogen fluoride must be removed from the mixture in order to effect transformation of substantially all of the hexafluoroarsenic acid to arsenic acid. Because hydrogen fluoride and water form an azeotrope when the weight percent of the hydrogen fluoride based on the total weight of the hydrogen fluoride and water is at least 38, arsenic acid has to be present in an amount sufficient to break the azeotrope. Generally, the amount of arsenic acid required to break the azeotrope between hydrogen fluoride and water is at least about 45 weight percent based on the total weight of the aqueous mixture. Any commercially available means for heating the final mixture may be used. For example, external heat may be supplied by a heater around vessel 30. Preferably, the temperature of the reaction mixture is about 120°C to about 130°C. If the reaction temperature is much below about 100°C, the amount of hydrogen fluoride removed is inadequate; the lack of appropriate materials of construction prevents the use of reaction temperatures much higher than about 150 °C. More preferably, the temperature of the reaction mixture is about 120°C to about 125°C. Most preferably, the amount of arsenic acid is at least about 50 weight percent and the temperature of the reaction mixture is about 120°C. The removal of substantially all the hydrogen fluoride also ensures the conversion of the hexafluorarsenic acid to arsenic acid which affects the final arsenic acid product purity. An inert gas as indicated by arrow 32 is passed through the heated mixture in a quantity sufficient to remove substantially all of the hydrogen fluoride in the heated mixture from the heated mixture. Any inert gas which is capable of removing substantially all of the hydrogen fluoride may be used for this step. Examples of useful inert gases include steam, air, nitrogen, methane, and carbon dioxide. Mixtures of inert gases may also be used. The amount of inert gas required is a function of the reaction temperature at which the inert gas is added. For example at a reaction temperature of 150°C, a minimum of about ten moles of inert gas per mole of hydrogen fluoride in the reaction mixture prior to gas addition is required in order to substantially remove all of the hydrogen fluoride. Because the partial pressure of hydrogen fluoride increases with temperature, higher reaction temperatures require less inert gas while lower reaction temperatures require more inert gas. Also, the degree and uniformity of mixing of the inert gas with the liquid reaction mixture are factors in determining the amount of inert gas to be used. The inert gas may be vented as indicated by arrow 34 to scrubber 20 and vented through arrow 22. The vaporized anhydrous hydrogen fluoride flows as indicated by arrow 36 out of vessel 30 and may be reused. The substantially pure aqueous arsenic acid which preferably comprises at least about 50 weight percent arsenic acid may be transferred as indicated by arrow 38 to storage tank 40. The separated arsenic acid may be reused. As an example, the separated arsenic acid may be used as a material in the production of a CCA wood preservative. Thus, the present invention provides a process for separating arsenic acid from a waste mixture comprising sulfuric and arsenic acids and water wherein the separated arsenic acid may be reused and the remaining sulfuric acid may also be reused. The present process is advantageous because it eliminates the current need for converting the hazardous arsenic waste to non-hazardous waste, stabilizing the non-hazardous waste, and landfilling the non-hazardous waste. The present invention is more fully illustrated by the following non-limiting Examples. EXAMPLES For the following Examples, the starting waste material had the composition in Table I below: COMPONENT WEIGHT PERCENT Hydrofluoric Acid <0.1 Arsenic Acid 9.1 Sulfuric Acid 55.9 Water 35 HAsF 6 <50ppm EXAMPLE 1 The starting waste having the composition above was mixed with water and contacted with sulfur dioxide gas under the pressure indicated in Table II below at ambient temperature. Agitation was used to ensure sufficient contact of the reactants. The percent conversion of As +5 to As +3 was determined by commonly known Ion Chromatographic analysis techniques. The results are also in Table II. Waste, grams 20 H 2 O, grams 20 H 2 SO 4 , added grams 0 SO 2 , kPa (psig) 362 (38) Time, hours 2.5 Conversion, % 70 The conversion of arsenic (V) to arsenic (III) in step (a) was 70 percent. EXAMPLE 2 (FOR COMPARISON) The following example illustrates the separation and recovery of arsenic solids (As 2 O 3 ). The results are in Table III below. Waste, grams 25 Start As, grams 1.2 As(III) Soluble, grams 0.36 As (III) Solids, recovered grams 0.77 As(III) Solids, % 64 Over 60% of the starting arsenic in the waste was recovered as arsenic (III) solids. Example 2 involved handling arsenic solids and filtration and washing steps which lead to hygiene and environmental problems. The following examples demonstrate a liquid reaction that avoids these problems. EXAMPLE 3 To avoid handling arsenic solids, the following example shows that the arsenic (III) solids may be dissolved in aqueous hydrofluoric acid and volatilized with heat as arsenic trifluoride by adding sulfuric acid and heating the solution to about 125°C. The reduction of the arsenic (V) to arsenic (III) was conducted as in Example 1. The results are in Table IV below. START Red Mix, grams 40 Solids, grams 0.67 H 2 SO 4 added, grams 44 HF added, grams 5 HF, % 5.7 Temperature, °C 125 Time, hours 1 RESIDUE As Removed, % 99+ HF, % <0.5 H 2 SO 4 ,% 82 This example demonstrates that the arsenic solids do not have to be separated by a means such as filtration. The hydrogen fluoride dissolves the solids which eliminates the hygiene and environmental problems inherent with solids handling. EXAMPLE 4 It was determined that any sulfur dioxide remaining in the solution after the reduction step will also be volatilized which will contaminate the product. The reduction of the arsenic (V) to arsenic (III) was conducted as in Example 1 above. The following example shows that a slow inert gas purge at room temperature will remove the sulfur dioxide but not the arsenic (III) even in the presence of hydrofluoric acid. The results are in Table V below. Reduced Mix, grams 35 STARTING Total Acid as H 2 SO 4 ,% 53 HF, % 6.4 As (III),% 2.2 SO 2 ,% 1.6 N 2 PURGE Time, Min. 30 PURGED MIX As (III), % 2.2 SO 2 , % <0.01 EXAMPLES 5 AND 6 Arsenic acid manufacturers react arsenic (III) oxide with nitric acid in the presence of potassium iodide as a catalyst. Thus, arsenic trifluoride, which is more reactive than the oxide, could also react in a similar manner. This reaction produces nitrogen oxide gases that must be contained. The nitrogen oxide gases and the potassium iodide could contaminate our product arsenic acid. The following examples show that hydrogen peroxide acts as an oxidizer for the arsenic (III) as arsenic (III) solids or trifluoride. Arsenic trifluoride reagent from a commercial supplier was added to a solution of 5% hydrogen peroxide. The results are set forth in Table VI below. EX 5 EX 6 ADDED H 2 O 2 Solution, grams 50 0.1 AsF 3, grams 2.8 --- Calc HF, % 2.4 --- As 2 O 3 , grams --- 0.13 FINAL As (III), % <0.02 --- HF, % 2.3 --- H 3 AsO 4 , % 8.5 --- As(V) found, grams 1.6 0.0995 As(V) Converson, % 100 100 The above examples show that hydrogen peroxide was an efficient oxidizer for the arsenic (III) to arsenic (V). At ambient temperature, the conversion was rapid and complete. The mixture was then boiled for one hour to try to remove the hydrofluoric acid. The volume was maintained by adding water. The results indicate that hydrofluoric acid was difficult to remove from weak mixtures of hydrofluoric and arsenic acid. EXAMPLE 7 This example demonstrates the decomposition of the hydrogen peroxide after generation of a mixture in the manner of Examples 5 and 6 above. Removal of the excess hydrogen peroxide was necessary for a pure arsenic acid product. The mixture was heated at the indicated temperatures and time to concentrate the solution and decompose the hydrogen peroxide. The results are in Table VII below. Start H 2 O 2 , % 4.5 H 3 AsO 4 , % 6.0 Heat Temperature, (°C) 95-100 Time, hours 2.5 H 2 O 2 , % 2.2 H 3 AsO 4 , % 8.75 Reheat Temperature, (°C) 130 Time, hours 4 H 2 O 2 , % 0.03 H 3 AsO 4 , % 16.6 This example indicates that some of the hydrogen peroxide is decomposed but not all in a reasonable time at about 100°C. At the higher temperature, the hydrogen peroxide was decomposed at the end of the heating period. The hydrogen peroxide may have been decomposed sooner but no samples were taken. EXAMPLE 8 The following example shows that the hydrofluoric acid can be removed from the impure arsenic acid mixture by increasing the arsenic acid concentration and temperature of the solution. Arsenic acid will break the hydrofluoric acid/water azeotrope. The mixture was heated and purged with nitrogen as an inert gas. Water was important for removing the hydrofluoric acid and preventing the arsenic acid from solidifying (converting to arsenic (V) oxide). The initial volume was maintained by water addition. A synthetic mixture of arsenic acid and hydrofluoric acid was used for the experiments. The results are in Table VIII below. Start: H 2 O, % 29.5 HF,% 8.7 H 3 AsO 4 , % 61.6 Conditions: Temperature,(°C) 120-140 N 2 Rate, cc/min 50-100 Time, hours 5 Final: H 3 AsO 4 ,% 51.3 HF,% 0.2 H 2 O,% 48.5 This example demonstrates that the hydrofluoric acid may be removed in a reasonable time at a reasonable temperature. It is felt that in actual commercial operation the time required for removal of the hydrofluoric acid would be reduced. EXAMPLE 9 The arsenic acid product was made starting from the original waste following through the process steps as described above. The final product had the composition of Table IX below: H 3 AsO 4 , % 46 HF, % <0.5 H 2 SO 4 ,% <1 HAsF 6 , ppm 19 Fe, ppm <50 Pb, ppm <50 This final product could be made more concentrated by stopping the water addition after the hydrofluoric acid and hydrogen peroxide were removed. This product may be used as a saleable product or as a raw material for the production of the CCA wood preservative.

Claims: A process for recovering arsenic acid from a starting mixture comprising sulfuric and arsenic acids and water comprising the steps of: (a) treating said starting mixture comprising at least 1 weight percent of arsenic acid, at least 1 weight percent of sulfuric acid and up to 98 weight percent of water, based on the total amount of said starting mixture, with a sulfur (IV) compound which will reduce the arsenic acid to arsenic (III) compound, under conditions sufficient to substantially convert said arsenic acid into arsenic (III) compounds, sufficient hydrogen fluoride being present in said starting mixture to substantially convert the arsenic (III) compound into arsenic trifluoride, and to provide a resulting mixture comprising said arsenic trifluoride, said sulfur (IV) compound, said sulfuric acid, and said water; (b) purging said resulting mixture with inert gas to substantially remove said sulfur (IV) compound from said mixture to provide a purged mixture comprising said arsenic trifluoride, said sulfuric acid, and said water; (c) adding sulfuric acid to said purged mixture, in an amount sufficient and at a temperature sufficient to substantially volatilize said arsenic trifluoride, wherein said volatilization substantially separates said arsenic trifluoride from said purged mixture; (d) reacting said separated arsenic trifluoride with oxidizing agent under conditions sufficient to substantially convert said arsenic trifluoride into arsenic acid to produce a final mixture comprising said arsenic acid, unreacted oxidizing agent, and said water; and (e) removing impurities from said final mixture, wherein said impurities comprise said unreacted oxidizing agent, to provide substantially pure aqueous arsenic acid. A process for recovering arsenic acid from a starting mixture comprising sulfuric and arsenic acids and water comprising the steps of: (a) treating said starting mixture comprising at least 1 weight percent of arsenic acid, at least 1 weight percent of sulfuric acid and up to 98 weight percent of water, based on the total amount of said starting mixture, with a sulfur (IV) compound which will reduce the arsenic acid to arsenic (III) compound, under conditions sufficient to substantially convert said arsenic acid into arsenic (III) compounds and to provide a resulting mixture comprising said arsenic (III) compound, said sulfur (IV) compound, said sulfuric acid, and said water; (b) purging said resulting mixture with inert gas to substantially remove said sulfur (IV) compound from said mixture to provide a purged mixture comprising said arsenic (III) compound, said sulfuric acid, and said water; (c) reacting said purged mixture with hydrogen fluoride, in an amount sufficient to substantially convert the arsenic (III) compound into arsenic trifluoride, and adding sulfuric acid to said purged mixture in an amount sufficient to substantially volatilize said arsenic trifluoride thereby substantially separating said arsenic trifluoride from said purged mixture. (d) reacting said separated arsenic trifluoride with oxidizing agent under conditions sufficient to substantially convert said arsenic trifluoride into arsenic acid to produce a final mixture comprising said arsenic acid, unreacted oxidizing agent, and said water; and (e) removing impurities from said final mixture, wherein said impurities comprise said unreacted oxidizing agent, to provide substantially pure aqueous arsenic acid. A process according to Claim 1 or 2 characterized in that said sulfur (IV) compound is selected from sulfur dioxide, sulfurous acid, sodium bisulfite, potassium bisulfite, ammonium bisulfite, sodium sulfite, potassium sulfite, and ammonium sulfite. A process according to Claims 1 or 2 characterized in that said sulfur (IV) compound is sulfur dioxide. A process according to any one of Claims 1 to 4 characterized in that the amount of said added sulfuric acid is at least 50 weight percent, based on the total amount of said purged mixture, and the temperature is at least 120°C. A process according to any one of Claims 1 to 5 characterized in that the oxidizing agent in said step (d) is hydrogen peroxide. A process according to any one of Claims 1 to 6 characterized in that said final mixture in said step (d), further comprises hydrogen fluoride, and said impurities in said step (e) further comprise said hydrogen fluoride.

Inventors: REDMON CHARLES LEWIS, SMITH ROBERT ANDREW, SUBBANNA SOMANAHALLI NARANAPPA, REDMON, CHARLES LEWIS, SMITH, ROBERT ANDREW, SUBBANNA, SOMANAHALLI NARANAPPA
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1994
Main classification: C01G28
IPC codes: C01B17, C01G28